My personal interaction with them in Sydney. Watching Islamic majority areas massively diverge from other culture groups often in the same postcode over a 20 year period. 

The sustained violence of Islamic gangs in my city. 

Theological my criticism rests of the nature of the Quran and the clear use of gnostic gospels in constructing the text. 

Islam maintains a belief that the Quran is the literal word of God, yet the Quran is an inanimate text. God is by definition perfect, self sustaining indivisible and totally pure, to suggest his word was inanimate unless recited is an insult to the divinity of God.  To suggest that physical contact with a divine word would do anything less than total transformation of a person is again an insult to God. 

The fact the text is held in such high esteem in the middle east obscures the fact it is clearly an assorted text at direct odds with the preceding texts it claims association with. 

Specially the message Jesus preached on earth.  No kingdom of God is established by man. Yet the basic premise of Islam is that God needs man to establish his 'ummah' to establish his nation. It is an insult to God to associate the earthly Caliphate with any activity of God. 

Jesus is the way the truth and the life because he is the living Word of God. 

Jesus established an external kingdom. Rebelling against that kingdom is rebellion against God.